But I use a prefab RTC board which already has pull up resistors and a rechargeable backup battery included.

Whatever I tried, it was just not working.

Double checking the wiring did not reveal any mistake made by me, so I decided to check the A4 en A5 ports. Guess what? I was using the SDA & SCL contacts that are positioned next to the Aref pin. Measuring those pin to A4 and A5, gave me the solution: SDA was connected to A4, but SCL is not connected to A5. BTW: SCL seems to be connected to nothing on my Arduino Uno R3.

Maybe it is coincidence, and am I in possesion of the only board with that fault, but it wouldn't do any harm to check your board if you're having troubles with I2C communication.

And many thanks for the library, it seems to work perfect, with very understandable commands.